Seismic surveying typically involves the utilization of a source of seismic energy and one or more arrays of seismic detectors. The arrays of seismic detectors are selectively positioned above an area of interest. Seismic waves generated by a seismic energy source are reflected and refracted by sub-surface geological formations and recorded by the seismic detectors.
The source of seismic energy typically utilized for land based operations may be an apparatus capable of delivering a series of impacts or mechanical vibrations to the surface of the earth or the detonation of a high explosive charge near the earth's surface. The resultant acoustic waves which are generated in the earth, including those which are reflected and refracted by the various interfaces within the formations of the earth, are converted by the seismic detectors into representative electrical signals. From these electrical signals, data may be deduced concerning the structure of the strata beneath the earth's surface.
Marine seismic surveying operates in much the same manner. An explosive device or vibration inducing compressed air system is typically utilized to generate seismic energy. The seismic energy then propagates as seismic waves into the earth formations below the body of water. Reflections and refractions of this seismic energy from the various strata within the earth are then detected by preferably a plurality of seismic detectors which are generally coupled together in one or more "live sections" of a sensor array which are towed behind a marine craft. It is common to employ a plurality of sensor arrays, each of which may contain over 10,000 seismic detectors. Further, multiple arrays are often towed in both a vertically spaced as well as in a horizontally spaced alignment.
The sensor array, or "streamer" generally includes a lead-in section, the live section and a tailbouy section. The lead-in section is positioned between the marine vessel and the first live section. When surveying with two or more streamers, barovanes may be inserted between the marine vessel and the lead-in section to achieve desired streamer spacing. The tailbouy lead-out is positioned between the free end of the last live section and the tailbouy.
Typically, "stretch" sections which are made of elastic materials, such as nylon or KEVLAR, are inserted between the lead-in section and the first live section and between the last live section and the tailbouy lead-out. The active section of the streamer generally consists of a fluid filled, elongated, flexible, tubular plastic jacket. The plurality of seismic detectors and other associated recording hardware are internally positioned along the length of the plastic jacket. Donut-shaped spacers are positioned at regular intervals within the plastic jacket. Each spacer fits snugly against the inside wall of the plastic jacket. Electrical and stress conduits, which are secured to and traverse these spacers, generally extend the full length of the live section.
A problem in all forms of marine seismic surveying operations is the presence of marine ambient noise. Different sources of marine ambient noise which occur during these operations include acceleration of the streamer during towing, vibration of the barovanes during towing, vibrations of the marine vessel (on board engines, generators, compressors, etc.), vibrations of nearby drilling rigs or passing ships, pressure variations caused by gravity waves propagating at the ocean surface, and turbulent, non-laminar water flow around the streamer. Of particular interest to the inventors is the general class of marine noise commonly referred to as "bulge wave noise". Bulge wave noise generally results from erratic lurching of the marine vessel, the barovanes and tailbouy. In relatively calm seas, bulge wave noise resulting from the marine vessel and tailbouy lurching can be somewhat minimized by controlling towing speeds. However, in rough sea conditions, such lurching may become so sever and uncontrollable that marine seismic surveying is interrupted until calmer seas return.
This lurching movement is communicated to the streamer which in turn causes the streamer to unpredictably accelerate and decelerate (hereinafter collectively referred to as "accelerations"). The resulting accelerations create extensional waves that propagate along the stress members of the streamer at about 1500 m/s. At each rigid connection between the stress members, spacers and other internal structure within the streamer, a number of lower velocity, mode converted energy waves are created that propagate within the streamer as well. Mode converted energy waves include very low velocity, perhaps 50 m/s, bulge wave energy that propagates within the streamer fluid and streamer skin. The extensionally-coupled bulge wave energy, along with other related forms of mode converted energy that may or may not result in a measurable deformation of the streamer skin, is recorded by the seismic detector within the streamer as noise (hereinafter collectively referred to as "bulge wave noise"). Once recorded by the seismic detector, the bulge wave noise severely contaminates the seismic reflection signals. Attempts to limit bulge wave noise levels have typically included the use of stretch sections and the application of low cut filters.
The stretch sections at both the front and back of the live section act as low pass filters that, under normal sea states, effectively attenuate or damp out undesired bulge wave noise above approximately 10 Hz. In higher sea states, the level of bulge wave energy increases at all frequencies, thereby contaminating the seismic signals even above 10 hz, often causing the seismic data collection operation to cease until lower sea states return. In addition, the historical trend of increasing seismic streamer length requires that additional stretch sections be used to achieve the same level of noise attenuation realized on shorter streamers. This has the undesirable effect of moving the near-offset detector group further away from the seismic source, resulting in a reduced ability to image the shallow geologic structures and possibly negatively effecting subsequent seismic data processing.
Attempts at attenuating the residual bulge wave noise below 10 Hz have traditionally required the use of low cut recording filters. However, the use of low cut recording filters has a major disadvantage in that the useable seismic bandwidth is also reduced, thus limiting the resolution of the recorded seismic reflection data. Therefore, there exists a need to attenuate bulge wave noise while minimizing resolution loss.